This invention relates to apparatus for generating a pulse width modulated wave which generates a pulse train whose pulse width gradually increases with time.
The principle of the apparatus for generating a pulse width modulated wave of the type just described is well known. That is, a combination of the outputs from respective stages of a first counter which counts a first clock pulse and a combination of the outputs from respective stages of a second counter which counts a second clock pulse having a frequency lower than that of the first clock pulse are compared with each other by a comparator which produces an output when both combinations coincide with each other. Furthermore, an R-S flip-flop circuit is provided which is set when the combination of the outputs from respective stages of the first counter becomes a predetermined combination, for example when the outputs of all stages are "0" and reset by the output of the comparator. With this connection it is possible to produce from the flip-flop circuit a pulse train whose pulse width gradually increases with time. In the prior art apparatus described above, the first and second counters have been constituted by serially connecting a plurality of T type flip-flop circuit. In such a counter, there is a time difference between the outputs of the lower stages and the outputs of the higher stages. For this reason, the comparator produces not only a desired coincidence output pulse but also a not desired pulse having a narrow pulse width thus making it difficult to obtain a desired pulse width modulated wave. According to another prior art apparatus, an AND gate circuit is connected between the input and the output of a flip-flop circuit which constitutes each counter and the operating times of respective flip-flop circuits are synchronized by a clock pulse, and such counters are termed synchronous counters. The latter apparatus can eliminate the defect of the former apparatus, but as it requires the AND gate circuits for synchronization, it is necessary to increase the number of elements for one bit of the counter, thus complicating the construction. This drawback becomes remarkable for a counter having a large capacity of bits.
Accordingly, it is an object of this invention to provide an improved apparatus for generating a pulse width modulated wave capable of efficiently generating a desired pulse width modulated wave with a simple circuit construction.